150 REPORT — 1859. 



provided has been found unnecessary, it has been deemed desirable to make 

 some additions to the instrument from time to time. 



The object-glass of the photoheliograph, it will be remembered, is of 3^ 

 inches clear aperture and 50 inches focal length, but the whole aperture is 

 never used ; it is always diminished more or less, and generally to about 

 2 inches, by a stop placed in front of the object-glass. The focal image of 

 the sun at the mean distance is 0"466 inch. The focal image is not, how- 

 ever, received directly on the sensitive plate, as in the case of taking lunar 

 and planetary photographs, but is enlarged before it reaches it by means of 

 a secondary combination of lenses (an ordinary Huyghenian eye-piece), which 

 increases the picture to about 4 inches in diameter, thus magnifying the image 

 about eight times linear, and diminishing the intensity of the light 64? times. 



The object-glass (made by the late Mr. Ross) is specially corrected to en- 

 sure the coincidence of the visual and chemical foci ; but, as might be anti- 

 cipated, the rays, after passing through the secondary lens, are in some degree 

 dispersed, and this coincidence of foci no longer exists. It required some 

 considerable time to determine exactly the position of the actinic focus ; 

 ultimately it was proved, after numerous trials, that the best photographic 

 definition is obtained when the sensitized plate is placed from -j^th to -^th of 

 an inch beyond the visual focus, and that this adjustment must be modified 

 to a slight extent according as more or less of the aperture of the object-glass 

 is employed. 



Difficulties of Photoheliography. — Whilst in lunar and stellar photography 

 many of the obstacles to be overcome arose from the deficiency of photo- 

 graphic power in the unenlarged focal images of those celestial objects, the 

 difficulties which have stood in the way of producing good sun-pictures arose 

 in a great degree from the incomparably greater brilliancy in the sun's image, 

 even when its intensity was considerably lessened by stopping off a large por- 

 tion of the object-glass, and magnifying the diameter of the image very 

 greatly. In order to overcome these obstacles, recourse was had at an early 

 period to the less sensitive media than wet collodion, such, for example, as are 

 used in the albumen and the dry collodion processes. None of these attempts 

 were, however, productive of sufficiently promising results to encourage the 

 pursuit of the trials in this direction, and I may mention that I made simul- 

 taneous experiments in taking unenlarged pictures in the focus of my reflector, 

 on dry collodion and albumen, with no better result. The surfaces in these 

 processes are indeed very rarely sufficiently free from impurities for the deli- 

 neation of such minute objects as solar spots, and the processes themselves 

 present disadvantages which render them inapplicable to photoheliography. 



After many unsuccessful trials a return was at last made to the collodion 

 process. Former experience having shown that the shortest exposure possible 

 with the means then at command produced only a solarized image, in which 

 all trace of the sun-spots was obliterated, recourse was had to the interposi- 

 tion of yellow glass between the principal and secondary object-glasses, with 

 the view of diminishing the actinic intensity of the sun's image; nevertheless 

 only burnt-up pictures were produced. 



Instantaneous Apparatus. — It will be evident, therefore, that, for the suc- 

 cessful employment of a medium so sensitive as wet collodion, it was absolutely 

 necessary to contrive some means for reducing the time of its exposure to the 

 sun's influence to an extremely small fraction of a second. Any apparatus 

 placed in front of the object-glass, it was conceived, would have the disadvan- 

 tage of cutting off the aperture by successive non-symmetrical portions, and 

 of producing an image less perfect than when the exposed portion of the ob- 

 ject-glass remained always concentric and circular. On the other hand, it was 



